A theoretical study of surface‐to‐borehole electromagnetic logging in cased holes

Abstract

A new electromagnetic logging method, in which the source is a horizontal loop coaxial with a cased drill hole and the secondary axial fields are measured at depth within the casing, has been analyzed. The analysis, which is for an idealized model of an infinite pipe in a conductive whole space, has shown that the casing and formation are uncoupled at the low frequencies that would be used in field studies. The field inside the casing may be found by first finding the field in the formation and then using this field as an incident field for the pipe alone. This result permits the formation response to be recovered from the measured field in the borehole by applying a correction for the known properties of the casing. If the casing response cannot be accurately predicted, a separate logging tool employing a higher frequency transmitter could be used to determine the required casing parameters in the vicinity of the receiver. This logging technique shows excellent sensitivity to changes in formation conductivity, but it is not yet known how well horizontal stratification can be resolved. One of its most promising applications will be in monitoring, through repeated measurements, changes in formation conductivity during production or enhanced recovery operations.

Subsurface electromagnetic measurement through steel casing

“…In this we agree with previous work on the casing problem done for other transmitter-receiver configurations (Augustin et al, 1989;Wu et al, 1994) where it was shown that the steel casing acts as a linear filter. Although it appears that the casing attenuation function is largely dependent on the casing induction parameter or its thickness/skin depth ratio, the formation signal can only be recovered properly if all the casing parameters are individually known with good accuracy.…”

“…Results of previous theoretical work and numerical calculations by Augustin et al (1989), Uchida et al (1991) and Wu and Habashy (1994) all pointed to the possibility of extracting a formation signal from electromagnetic measurements through casing. In particular, the Augustin results showed that the casing acts as a strong low pass filter.…”

Steel-cased wells in 3-D controlled source EM modelling

“…Wait & Hill (1973) considered a long vertical extended cylinder in a homogeneous halfspace. Augustin et al (1989) investigated how EM measurements could be carried out behind a metal casing.…”

Electromagnetic Fields Due to a Loop Current in a CasedBorehole Surrounded by Uniform Whole Space

“…(r) the electric permittivity, *(r) the magnetic permeability, and J,(r) the source current distribution expressed as (Augustin et al, 1989) where I(=) is the amount of the current impressed, and ( I is the radius of the current ring.…”